Machine and method for crust piercing and feeding of molten electrolytic baths



March 5, 1968 J CHAMBRAN 3,

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS Filed Sept. 23, 1964 16 Sheets-Sheet l INVENTOR. Jacques Chambran March 5, 1968 J. CHAMBRAN 3,372,106

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS Filed Sept. 25, 1964 l6 Sheets-Sheet 2 March 5, 1968 MACHINE Filed Sept. 23, 1964 J. CHAMBRAN 3,372,106 AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS l6 Sheets-Sheet 3 March 5, 1968 J. CHAMBRAN 3,

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS l6 Sheets-Sheet 4 Filed Sept. 23, 1964 March 5, 1968 J. CHAMBRAN 3,

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING 7 OF MOLTEN ELECTROLYTIC BATHS Filed Sept. 25, 1964 16 Sheets-Sheet 5 1 s s F Fig. 8

I 362 as: 1.11 H'w 5 call g: 2 358 i 320) 351 352) 359 Fig.7

March 5, 1968 J. CHAMBRAN MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS l6 Sheets-Sheet 6 Filed Sept. 23. 1964 527 529 53| l I l Fig. 9

March 5, 1968 J. CHAMBRAN 3,372,106

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS Filed Sept. 25, 1964 16 Sheets-Sheet 7 I 10o: ma

1 541 L 1015 104 {j l @I] L, 5 5 1012 1055 I March 5, 1968 J. CHAMBRAN MACHI NE AND METHOD FOR CRUST PI ERC ING AND FEEDING OF MOLTEN ELECTROLYT IC BATHS l6 Sheets-Sheet 8 Filed Sept. 23, 1964 March 5, 1968 J. CHAMBRAN 3,

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS Filed Sept. 23. 1964 16 Sheets-Sheet 9 1021 1014 1011 mo 5" 1m 5 m Fig.14

March 5, 1968 J. CHAMBRAN 3,372,106

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS Filed Sept. 25, 1964 16 Sheets-Sheet l O 1m 41 m9 J W m 1 1040 401 a 592 M 1 59s 7 March 5, 1968 J.CHAMBRAN 3,

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS Filed Sept. 23, 1964 16 Sheets-Sheet ll March 5, 1968 J. CHAMBRAN 3, 7

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS I Filed Sept. 23. 1964 16 Sheets-Sheet 12 March 5, 1968 J. CHAMBRAN 3,

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATES Filed Sept. 23, 1964 l6 Sheets-Sheet 13 his 1089 4001. 744 1995 m m D 10s;

March 5, 1968 J. CHAMBRAN 3,372,106

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS Filed Sept. 23. 1964 16 Sheets-Sheet 14 P G77 M1 4082 F G74 1108 J m @112 j M08 nos I ms l9 K mm 1: 575 578 March 5, 1968 J. CHAMBRAN 3,372,106

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS Filed Sept. 23, 1964 16 Sheets-Sheet 15 March 5, 1968 J. CHAMBRAN 3,

MACHINE AND METHOD FOR CRUST PIERCING AND FEEDING OF MOLTEN ELECTROLYTIC BATHS Filed Sept. 23, 1964 16 Sheets-Sheet 16 ii ited States Patent 25 Claims. (61. 204-245) ABSTRACT OF THE DISCLOSURE A machine for feeding powdered alumina into molten baths having a crust on the surfaces thereof and through which anodes extend into the baths for the production of aluminum by electrolysis and in which a plurality of such baths are aligned in side by side spaced apart relation, said machine comprising a carriage and means for displacement of the carriage from a starting position to a position in front of the baths, a piercing hammer mounted on the carriage for displacement vertically between raised and lowered positions and means responsive to the resistance to penetration of the hammer through the surface of the bath for initiating the operation of the piercing hammer to form an opening through the crust, a hopper on the carriage for the storage of alumina and means for discharging alumina from the hopper into the opening pierced by the hammer, said machine embodying sequencing and control means responsive to the position of the carriage relative to the baths and responsive to the position of the piercing hammer between raised and lowered positions adapted to (1) displace the carriage to a position in front of the anode of the bath to be fed, (2) lower the piercing hammer responsive to positioning of the carriage to pierce the crust to form an opening through the surface of the bath, (3) raise the piercing hammer from the bath, (4) displace the carriage a short distance responsive to movement of the hammer towards raised position, (5) pierce the bath again and continue the operation of steps (2), (3) and (4) until a number of openings have been provided in the surface of the bath, (6) feed aluminum from the hopper onto the surface of the bath, and (7) displace the machine to starting position.

This invention relates to a machine for feeding raw materials to baths for fusion electrolysis and relates more particularly to a machine for use in the production of aluminum by electrolysis to pierce the crusts and to feed powdered ingredients into the molten production bath.

In accordance with past practices, the crusts formed on the surfaces of the molten baths of alumina were perforated manually at portions selected by experience by the operators for purposes of gaining access to the interior of the bath for feeding alumina and the like powdered raw materials into the molten bath.

In the French Patent No. 1,245,598, description has been made of the machine for piercing and distributing alumina into electrolytic baths. The operation as described in the aforementioned patent is characterized with a number of deficiencies from the standpoint that it is incapable of controlling the piercing and the feeding operations uniformly to distribute the introduction of alumina over the surfaces of the molten baths. It is believed that the problems raised by such machines can be obviated by a machine which operates automatically to effect piercing and feeding operations in compliance with a set program and preferably a predetermined program.

the head of the series of baths in question, filling of the It is an object of this invention to produce a machine which operates automatically to pierce and feed baths for fusion electrolysis and in which the machine is capable of controlling feed on a predetermined program preceded by a piercing operation.

More specificially, it is an object of this invention to produce a machine which is automatic in operation, which can effect a piercing operation to perforate the crusts formed on surfaces of molten electrolytic baths, which can effect a feeding operation to introduce particulate feed material through the perforations formed in the crusts, which is programmed to carry out the perforating and feeding operations at points regularly distributed over the surfaces of the baths, which is capable of operation without human intervention automatically to control the piercing and feeding operation and the distribution of feed into the baths, which is relatively simple in construction and easy in operation, which is capable of steady operation over long periods of time to maintain the health of the baths for the production of aluminum by electrolysis, and which is flexible in operation to control the feed to any number of baths making up the pot line.

The machine, embodying the features of this invention, will hereinafter be described with reference to a specific use in the production of aluminum by electrolysis wherein a plurality of electrolytic cells are arranged in spacedapart relation along a line to form, what is referred to in the trade as a pot line, and in which the baths are fed with powdered alumina for reduction to a molten state by the heat generated during passage of electrical current from anodes, which extend downwardly into the molten bath, to the cathodes whereby the alumina is reduced to metallic aluminum which collects as a molten metal at the bottom of the bath for removal as product while the molten material at the surface of the bath is exposed to ambient temperature to form crusts which are required to be pierced for feeding alumina into the bath.

The machine according to the invention comprises in combination a carriage forming a general supporting structure and comprising means which allow it to be displaced in front of the baths in question, the piercing and feeding operations taking place during the displacement; a piercing hammer controlled by a fluid under pressure and mounted on a jack which allows a vertical translatory movement to be imparted to it, the two pieces of apparatus being fed in parallel by the same under pressure fluids so that the hammer does not begin to function until the resistance of the crust arrests the descending translatory movement; an alumina hopper equipped with a dosing receptacle and a valve enabling the contents of the receptacle to be discharged, the hopper being set back from the piercing hammer in the direction of displacement of the machine during the piercing operations; a sequence mechanism, controlled by a time switch, and positioning members of which some are located on the bath to determine the position of the carriage and others on the piercing hammer to determine whether it is in the raised or lowered position, and which actuate proximity detectors mounted on the apparatus. It carries out the following operations:

(a) While the machine is in its starting position, at

alumina hopper from a silo provided for the purpose;

(b) Starting of the machine until it is in front of the 3 discharge into the bath of the alumina contained in the dosing receptacle;

(e) Repetition of the above piercing and feeding operation opposite a certain number of anodes; and,

(f) Return to the starting point after a given number of feeding operations, and refilling of the hopper from the silo disposed for this purpose.

In a preferred embodiment, the sequence mechanism also has a supplementary member controlled by an anode known as the pilot anode of the bath, which blocks the alumina-feeding mechanism as soon as the dissolved alumina content of the liquor reaches a value of between 4 and 7 percent, and which frees this mechanism as soon as the content drops to a value between 1.5 and 4 percent.

In a special embodiment of the invention, each series of electrolytic baths is equipped with two automatic machines located one on each side of the baths. One proceeds with the piercing operation as a function of a definite program whereas the other is kept in reverse to answer summons triggered off by the phenomenon of anodic polarization, known as burning of one of the baths or even by preburning as will be explained hereinafter. The functions of the two machines is reversed after a given period of time or a given number of runs.

The foregoing objects and other objects and advantages of the invention will hereinafter be described and for purposes of illustration, but not of limitation, embodiments of the invention are shown in the accompanying drawings in which:

FIGURE 1 is a diagram of the piercing cycle in the system of baths with prefired anodes;

FIGURE 2 is a perspective view of the moving portions of the machine embodying features of this invention;

FIGURE 3 is a digrammatic elevational view showing the winch assembly;

FIGURE 4 is a diagrammatic elevational view of the assembly which includes the pneumatic hammer and means for its control fixed on the framework of the machine;

FIGURE 5 is a diagrammatic view in elevation of the pneumatic circuit controlling the piercing hammer and the alumina feed;

FIGURE 6 is a diagrammatic elevational view of a means for feeding alumina;

FIGURE 7 is a schematic view of a differential doubledbodied jack used to pierce the top of the baths or to carry out the piercing operation in more than two lines in front of the anodes;

FIGURE 8 is a diagram which shows the piercing points at the head of a first bath anode;

FIGURE 9 is a longitudinal diagram which shows the position of the proximity detectors on the machine and their energizing members on the bath and it relates to a series equipped with two machines located on each side of the baths;

FIGURE 10 is a transverse diagram of the elements in FIGURE 9 to show the position of the essential members of two machines serving the same series of baths;

FIGURES 11 to 25 are partial diagrams of the controlling sequencing apparatus in which the elements are represented by numerals within the range of 500 to 733 and in which the leads are represented by numerals above 1000.

In FIGURES 11 to 25, the partial diagrams can be connected by joining the leads bearing the same numerals. FIGURE 11 shows the means for signaling the position of the carriage and of the hammer 540. FIGURE 12 shows the starting means 550. FIGURE 3 shows the timing means 560. FIGURE 14 shows the memory of the piercing assembly 570. FIGURE 15 shows the memory of effective piercing 580. FIGURE 16 shows the means controlling unpacking after burning and preburning 590. FIGURE 17 shows the memory of unpacking after burning 600 in which unpacking refers to the elimination of the effect of an anodic polarization. FIGURE 18 shows the generator of the impulse which starts the piercing operation 610. FIGURE 19 shows the memory of the return of the carriage 620. FIGURE 20 illustratesthe record of piercing 630, FIGURE 21 shows the counter F 740. FIGURE 22 shows the coincidence circuit 660 FIGURE 23 shows the pairs counter 670. FIGURE 24 shows the record of returns 680. FIGURE 25 shows the means controlling the electrical valves and the motors 710.

In FIGURES 11 to 25, the logical members are represented by conventional signs, since the details of each member are well-known to those skilled in the art. On each member, an energizing input, that is to say, an input causing a signal to appear at the output of the member, is represented by an arrow whereas a prohibition or blocking input, i.e., an input causing the signal to be suppressed at the output of the member, is represented by a dot. A member which recurs frequently in the diagrams is the OR circuit, for example 542 in FIGURE 11. The output of this member emits a signal each time at least one of its inputs in energized. The member is represented by a circle with the inputs ending in a segment of a straight line at a tangent to the circle and the output diametrically opposed to the point of contact of thetangent. Similarly, the AND circuit occurs frequently, for example 543 in FIGURE 11. The output of this member emits a signal each time all its inputs are energized simultaneously. It is represented by a square or rectangle with the inputs ending at one of its sides and the output or outputs extending from the opposite side. An OR circuit comprising at least one blocking input is sometimes described as an OR-NOT circuit, while an AND circuit, comprising at least one blocking input, is similarly sometimes called an AN-NOT circuit.

The operation of the automatic machine is based on a certain number of principles which applicants have established either as a result of experiments accompanied by exact measurements and carried out on industrial baths selected from a series, or as a result of experiments of long duration carried out on entire series of baths. These principles are as follows:

The alumina content of the liquor must not be substantially lower than a minimum content of the order of 2 percent;

The energy yield of the operation is increased when the alumina content of the liquor is increased from this minimum value; I

The alumina content of the liquor must not be substantially greater than a maximum content, for example of the order of 7 percent; and

When said minimum content is detected, the crust is pierced and the liquor fed with alumina, preferably in several stages, until the alumina content of the liquor substantially reaches the maximum.

Observation of these principles results in a saving in electricity and in the consumption of fluorine containing products in the electrolytic liquor. If the alumina content is kept between two limits in this way, it becomes possible first to prevent burning and second to prevent the addition of surplus alumina to the liquor from resulting in deposits of this oxide of the cathode in the bath. These deposits have been found to be particularly harmful both to the normal course of the electrolytic operation and to the preservation of the cathode. The said regulation of the alumina content also permits a substantial increase in the means dissolved alumina content of the liquor which results in an improvement in the energy yield from electrolysis.

In French patent application No. 909,846, filed Sept. 19, 1962, under the title Improvements to Igneous Electrolysis of Alumina and in the first addition to this application, filed Sept. 3, 1963, under No. 946,411, applicants described a process for detecting the dissolved alumina content of the electrolytic liquor. In this process, a current impulse is passed through an anode which is known as the pilot anode and the base of which is of small surface relatively to the total anodic surface. The impulse is such that the base is crossed by a current of greater density than that d of the current passing through the anodic surface of the bath during normal operation, preferably from 1.5 to times this density d and from 1 to 10 A./cm. more particularly from 2 to 6 A./cm. The voltage of the pilot anode is measured for the duration of this current impulse. The impulse is repeated systematically until the voltage of the pilot anode exceeds a value, known as the volt-rise value, which is greater than the maximum value observed during the preceding impulses, and preferably between 1.05 times and twice this value. By means of the volt-rise at the pilot anode, the time is detected when the dissolved alumina content of the liquor drops below a given threshold (limit value) which depends on the impulse density chosen. A so-called preburning signal is sent to the piercing and feeding machine as soon as polarization takes place for a current density, chosen at random within the limits defined above, corresponding either to the minimum or to the maximum dissolved alumina content of the bath to be detected.

The machine according to the invention is suitable to carry out this process or any other process for prevent-ing burns. The machine thereafter operates as follows. In normal operation, if no signal is emitted by the pilot anode, the machine pierces and provisions in a set rhythm slightly faster than the rhythm of normal consumption of alumina. The alumina content of the liquor thus increases and, after a certain number of piercing and feeding operations, reaches the set upper limit. The pilot anode then emits a signal to prohibit feeding. The machine continues to pierce the crust but no longer feeds the bath, and the alumina content of the liquor drops and finally reaches the lower limit. The signal prohibiting feeding is then withdrawn, and feeding with alumina is continued.

The piercing of the crust may be discontinuous with the whole of each bath being pierced before the next bath, or continuous with each bath being pierced over a fraction of its length, for example over the length of one anode in the case of multiple-anode baths, prior to the next bath being pierced over the same fraction of its length. Piercing may be carried out in one or more lines, but applicants have found that for present day industrial baths with prefired anodes or anodes of the Soederberg type, the best results are obtained by piercing along two lines.

The number of machines used per series of baths depends on the size of the series. In the case of short series with a small number of baths, one machine is sutficient. It will then turn back on itself at the end of the series so as to serve both sides of the series in succession. In the case of series of medium length, two machines Will be used, each serving one side of the series. The turn at the end of the series is thus avoided. In the case of very long series, several machines may be provided on each side.

By way of example, a description will now be given of a machine designed to serve one side of a series of fifteen SO-kiloamp baths with prefired anodes for alumina electrolysis. Each bath has 12 anodes, six at each side. The machine described carries out its useful work, piercing and feeding during displacement in a certain direction, which is always the same, and is described as the working direction. It does no work during its displacements in the reverse direction, described as the returning direction. This results in simplification of the controlling electronic circuits, although any other arrangement is possible.

The elementary piercing operation, known as the piercing cycle, comprises eight piercing points, four points 1, 3, 5, 7 in the front position, i.e., in the vicinity of the anode, and four points 2, 4, 6, 8 in the rear position, i.e., near the wall of the bath. The piercing cycle is repeated in front of each anode. When the machine is being displaced in its working direction and the hammer arrives at right angles to the anode 21 to be pierced, it goes into the front position above the point 1. It descends and strikes as soon as it meets resistance. As soon as the hammer has reached the low position, or if it fails to reach the low position, after a period of time, t it rises again. At the end of its stroke in the raised position, the hammer passes into the rear position below the point 2, redescends and pierces the crust. The machine starts again, stops, pierces the points 3, then 4, and so on. When the point 8 has been pierced, the machine starts up again in the working direction 20, and either recommences the cycle in front of the following anode 22 in the same bath, in the case of discontinuous piercing, or reaches the following bath and repeats the operation in front of the corresponding anode in the case of continuous piercing. In both cases, when the hammer has pierced the point 8, then covered a distance I corresponding to a travelling time t and thus arrives at 9, the dosing receptacle of the feeding hopper opens so that the pierced zone is covered with alumina.

The whole piercing cycle is repeated in front of each anode.

The moving part of the machine consists of a frame comprising a tubular frame and an upper frame supporting the controls. The lower part of the tubular frame carries a wheel 111 which freewheels along the floor of the electrolytic workshop, while the upper frame carries two driving wheels 121 and 122 controlled by the motor 124.

The upper frame 120 also carries the various pieces of apparatus which will now be described. Fixed on the frame 120 are the winches and their mechanism, the assembly bearing the reference 200 and comprising a winch 210 for a flexible lead 211 bringing the compressed air necessary for the pneumatic controls, and its controlling motor 212. The flexible lead 211- passes over pulleys 213 (not shown in FIGURE 2 for reasons of clarity) and 214. A winch 220 for the electric cable 221 feeds the pieces of apparatus with electricity, and its controlling motor 222. The cable 221 passes below the drum of the winch 210 into a guiding spout 223.

Fixed below the frame is all the mechanical, electrical and pneumatic apparatus proper, that is to say the assembly 300 comprising the pneumatic hammer and its control means, the alumina-feeding assembly 400 and the sequence control assembly 500.

The assembly consisting of the pneumatic hammer and its control means comprises the hammer proper 301, of which the pick 306 is extended by a rod 307 adapted to slide in a cylindrical space 304 in the body of hammer. This space opens into a compressed air chamber 303, but communication between these two spaces may be interrupted by means of the obturator 305. The rod 307 contains an indentation 308 which restricts the stroke of the pick 306 by means of the bush 309 rigidly fixed to the body of the hammer. The hammer may perform a rising or descending vertical translatory movement by means of a differential jack having two outlets 310, the shaft 311 of which is linked to the upper frame 120 by means of a Cardan suspension 302 (not shown in detail in FIGURE 5), and the body 302 of which is rigidly connected to the piercing hammer 302. The volume 314 of the jack 310 above the small face of the piston 313 is in constant communication with the supply for the general pneumatic circuit 331 fed by the cable 211, while the volume 315 below the large face of the piston communicates with the pneumatic circuit by means of a valve with electrical controls or electric valve 332 also controlling the piercing hammer 301, When the jack 310 is open, the hammer is in the low position.

The hammer operates as follows. When the electric valve 332 is not energized, the volume 314 is in com- 7 munication with the circuit 331, thus keeping the jack closed. The hammer is in the raised position. When the electric valve 332 is energized, the volume 315 is also put into contact with the circuit 331. The force applied to the large face overcomes the force applied to the small face of the piston 313 and the jack opens. However, the

pressure of the pneumatic circuit, transmitted to the space 303, repels the obturator 395 and thus closes the inlet to the space 304. As soon as the pick encounters resistance, the rod 307 rises in the space 304, repels the obturator and thus allows the compressed air present in the space 303 to act on the rod 367. The pick 3% is repelled and the hammer strikes a blow. The obturator 305 is again repelled and the operation recommences. When the electric valve 332 is no longer energized, the hammer stops striking and ascends to the raised position. The jack 310 may either be rigidly connected to the hammer or independent thereof.

The body 312 of the jack 310 carries a plate 533 while the frame 120 carries two devices for detecting the position of the hammer, namely, the raised position detector 516 which is opposite the moving plate when the hammer is in the corresponding position, and the lowered position detector 517 which is opposite the same plate in the corresponding position of the hammer. The two detectors provide references for setting the position of the hammer, the lower position detector cutting oft" the supply of electricity to the electric valve 332 as soon as the hammer comes into the lowered position and thus causing the hammer to move into the raised position.

Within the vertical plane perpendicular to the axis of the series of baths, the hammer can take up several inclinations by the action of a differential jack with two outlets 320, in which the shaft 321 of the piston 323 is connected to the body 312 of the jack 310 carrying the hammer by a resilient means such as a spring jack 326 which enables the hammer to take on an inclination greater than that resulting from the position of the jack 321i, and by the action of a lateral force directed towards the axis of the bath. The spring jack 32s may be incorporated in the output shaft 321 of the differential jack 320 as shown in FIGURE 4. The volume 324 of the jack 320 located in front of the piston 323 communicates directly with the general pneumatic circuit 331, while the volume 325 to the rear of the piston 323 communicates with the circuit 331 through the electric valve 333. The body 322 of the jack 320 is fixed onto the tubular frame 111') by means of the jack-holding support 112.

The device operates as follows. When the electric valve 333 is not energized, only the small face of the piston 323 is fed The jack 321i is closed and the piston is in the rear position. The hammer then pierces a point such as 2, 4, 6 or 8. By means of the jack 326, it can slide along the ramp and scratch the latter. When the electric valve 333 is energized, both faces of the piston 323 are fed but the force applied to the large face, i.e., the rear face, overcomes the force ap lied to the small face. The jack 320 opens and the hammer moves into the forward position. It can then pierce a point such as 1, 3, 5 or '7.

The hammer is guided in this movement by a hammer guide comprising two blades 341 and 342 of which one end is articulated to the hammer 3111 and the other end to a plate 343 which is fixed to straps 344 articulated at 345 to a support 123 rigidly connected to the upper frame 120. The straps also support the jack 320326 at 346 and possibly a jack 350 which will be described hereinafter. This arrangement gives great flexibility t0 the suspension of the piercing hammer.

The alumina feeding assembly 4% comprises a hopper 410, a deformable diaphragm valve 420 and a dosing receptacle 430 of specific capacity.

The hopper 410 comprises at least one air guide 411. The function of the guide is to encourage the alumina to pass into the valve 420, and it comprises a cloth or porous slab 413 through which a current of gas is passed, for example a current of air taken from the pneumatic circuit 331. The hopper is completed (FIGURE 6) by a device 414 which shuts oif the inflow of alumina as soon as the hopper is full. The device illustrated comprises a small electric motor 415 driving the propeller 416. When the alumina reaches the level of the propeller, the moveu ment of the latter is braked with the aid of any mechanical or electrical means. This causes a relay controlling an electric valve to close and thus arrests the inflow of alumina. Feeding may be automated by a device 440 comprising the following:

Fixed on the hopper 411?, the relay 441 controlled by the device 414 when the hopper is full, the relay being connected to an energizing winding DP Ex 442 by the lead 1172, and a plate 443; and,

Fixed on the storage silo 447 which feeds the hopper 411%, a detector DPll 443 energized by DP Ex, another detector DPZ 44-4 and the two relays 445 and 446.

The relay 445 is energized by Dll whereas the relay 446 is energized by DPZ but by means of the movable relay 445 contact The movable contact of the relay 446 cts, by means of the lead 1173, on the electric valve (not shown) controlling the discharge of alumina from the silo 447 into the hopper 418.

The signal TR transmitted by the lead 1172 constitutes a prohibition in the return memory 620 described hereinafter.

The valve 429 comprises a diaphragm 421 behind which compressed air taken from the pneumatic circuit 331 can be sent into the space 422 by means of the electric valve 423. The outer wall of the valve contains a small aperture through which compressed air can fiow. The valve remains thus closed only so long as pressure is applied at 422.

The dosing receptacle 431 comprises the bottomless receptacle 431 and the rocking base 432 actuated by the pneumatic jack 433 which is connected to the electric valve 423 in parallel with the valve 420.

The feeding device 400 operates as follows. Assuming that the hopper 410 is full, the air guide 411 which is still fed drops the alumina into the bottom of the hopper. When the electromagnet of the electric valve 423 is not energized, the compressed air presses neither into the space 422 in the valve 420 nor into the jack 433. The receptacle 43% is closed, the valve 421) is open, and the receptacle is filled with alumina. When the electromagnet of the electric valve is energized, the compressed air passes both into the space 422 in the valve 420 and into the jack 433. The valve 430 is closed while the dosing receptacle is open. The dose of alumina is discharged onto the electrolytic liquor in the bath above which the machine is standing.

The apparatus described enables one side of the electrolytic baths in the series to be pierced longitudinally along two lines and the pierced zones to be fed with alumina following the cycle shown in FIGURE 1.

If it is desired to pierce along one line only, the jack 320 is eliminated.

If, on the other hand, it is desired to pierce along more than two lines or else to pierce the heads of the baths, then a differential double-bodied pneumatic jack 350 is placed in series with the jack 320.

The jack 351) comprises two jack bodies 351 and 352 separated by a wall 353, and two pistons 354 and 355. The shaft 356 of the first piston is connected to the jack 320, while the shaft 357 of the second piston is connected to the holding support 112 fixed to the frame. The two outer chambers 353 and 359 of the two jack bodies are permanently connected to the general pneumatic circuit 331, and the two inner chambers 360 and 361 of these two bodies are connected to the same circuit 331 by means of two electric valves 352 and 363, respectively.

When neither of the two electric valves is energized, only the outer chambers 358 and 359 are under pressure. Each of the two pistons is at the bottom of its respective chamber in the position shown in FIGURE 7. The jack 320 is in the normal working position for the piercing cycle shown in FIGURE 1. When only one of the electric valves is energized, the corresponding piston comes to the top of its chamber, and the hammer is in its first piercing position. When both electric valves are ener- 

